A reduced pressure electron beam (RPEB) process is being developed in the UK by ‘The Welding Institute’ (TWI) for the manufacture of thick section plate, forging and pipe welds. Potential gains include minimal distortion, fewer weld defects and rapid production times, compared to conventional arc welding processes. To date, RPEB welding equipment and parameters have been successfully developed and applied in a sealed chamber under partial vacuum (∼1mbar), to produce 1-pass seam welds in low alloy steel plates and forgings. Rolls-Royce commissioned TWI to produce RPEB welds in SA508 Grade 3 Class 1 steel forgings of between 100mm and 160mm thickness, as part of a feasibility study for nuclear pressure vessel applications. This paper presents results of micro-structural examinations, material property tests and residual stress analyses of RPEB welds in SA508 Grade 3 steel forgings, both in the as-welded condition and after post weld heat treatment (PWHT). This data was required in order to assess the structural integrity of the weldment. A narrow uniform fusion zone approximately 10mm wide and 3mm deep heat affected zone (HAZ) was produced. High hardness levels were measured in the weld HAZ, but the application of PWHT at 600°C had a beneficial tempering effect, reducing the maximum hardness to below 300Hv. A 3D finite element model was used and deep hole-drilling measurements were independently performed, to determine welding residual stress distributions. In both cases yield magnitude tri-axial tensile stresses were evaluated in the centre of the weld and adjacent HAZ, of up to +600MPa. This result is as expected since the centre region is the last to solidify and cool, with a high degree of restraint to weld shrinkage. However effective stress relaxation occurs during PWHT, mainly due to creep, reducing the maximum residual stress to about 100MPa (or 20% yield strength). This feasibility study has shown that RPEB welding is a viable method for fabricating large pressure vessels in low alloy steels. Sound joints can be produced in sections up to 160mm thick in a vacuum chamber. Further development work is being done by TWI in order to apply the technique out-of-chamber using a local vacuum seal.
Welding of thick section components such as pressure vessels used in the nuclear industry is traditionally performed using arc welding techniques, requiring multiple weld passes, with interstage non-destructive examination (NDE) and preheating of the component to reduce the risk of hydrogen cracking. Electron beam (EB) welding offers the opportunity to weld thick section components in a single pass and negates the need for interstage NDE, resulting in saving significant time and cost in the fabrication of nuclear vessels. Furthermore, elimination of the preheat step may be possible since the EB process is carried out in a vacuum environment. However, due to the physical size and geometry of nuclear vessels, traditional vacuum chambers would be prohibitively expensive when considering the low volume of output in the nuclear industry. Rolls-Royce and TWI are working on a local vacuum, or 'out of chamber', EB welding system, which eliminates the need of such a vacuum chamber and brings the vacuum to the component, thus making EB welding of nuclear vessels more practical and financially viable.
Boiler and steam piping components in power plants are fabricated using creep strength enhanced ferritic (CSEF) steels, which often operate at temperatures above 550°C. Modification of alloy content within these steels has produced better creep performance and higher operating temperatures, which increases the process efficiency of power plants. The improved materials, however, are susceptible to type IV cracking at the welded regions. A better understanding of type IV cracking in these materials is required and is the basis of the Technology Strategy Board (TSB) UK funded VALID (Verified Approaches to Life Management & Improved Design of High Temperature Steels for Advanced Steam Plants) project. In order to study the relationship between creep performance and heat input during welding, several welds with varying amounts of heat input and resultant HAZ widths were produced using the electron beam welding process. The welding parameters were developed with the aid of weld process modeling using the finite element (FE) method, in which the welding parameters were optimized to produce low, medium and high heat input welds. In this paper, the modeling approach and the development of electron beam welds in ASTM A387 grade P92 pipe material are presented. Creep specimens were extracted from the welded pipes and testing is ongoing. The authors acknowledge the VALID project partners, contributors and funding body: Air Liquide, Metrode, Polysoude, E.ON New Build & Technology Ltd, UKE.ON, Doosan, Centrica Energy, SSE, Tenaris, TU Chemnitz, The University of Nottingham, The Open University and UK TSB. Paper published with permission.
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